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Microsoft Corp. on Wednesday warned of several flaws in its ubiquitous Office products, the most serious of which could allow an attacker to take control of a user’s computer.

Deemed “critical” is a flaw in Visual Basic for Applications (VBA), a technology that is part of Microsoft Office products and used to run customized applications on top of Office. A flaw exists in the way VBA checks the properties of a document when it is opened in an Office application, potentially allowing an attacker to run code on a victim’s computer, Microsoft said in security bulletin MS03-037. (See: www.microsoft.com/technet/treeview/default.asp?url=/technet/security/bulletin/MS03-037.asp)

To exploit the flaw, an attacker would have to get a victim to open a specially-crafted document. This could be any document type that supports VBA, including Word, Excel or PowerPoint documents, Microsoft said. Also, if Word is used as the e-mail editor for Outlook, the default setting in Office XP/2002, an attacker could strike via e-mail. The attack would only be successful if the recipient forwards or replies to the e-mail message, Microsoft said.

The VBA flaw affects Access, Excel, PowerPoint and Word in Microsoft Office 97, 2000 and XP/2002 as well as Word 98, Project 2000 and 2002, Publisher 2002, Visio 2000 and 2002, Works Suite 2001, 2002 and 2003 plus several Microsoft Business Solutions products that also include VBA, Microsoft said.

Microsoft urges users of the affected products to patch at their earliest available opportunity. Users of more than one affected product may have to apply multiple software fixes, Microsoft said.

In addition to the VBA flaw, Microsoft also warned of three more security vulnerabilities in Office products, two carrying an “important” severity rating and one “moderate.”

Macros are executable code meant to automate commonly-performed tasks and can perform any action a user can on a PC. An attacker could create a malicious document that automatically runs a macro when opened, Microsoft said.

The flaw affects Word versions 97, 98, 2000 and XP/2002 as well as the Works Suite versions 2001, 2002 and 2003, Microsoft said.

The converter does not correctly validate certain parameters when opening a WordPerfect document. As a result, an attacker could craft a special WordPerfect document that would allow code to run on a computer when opened with an application that uses the converter, Microsoft said.

The last of the four flaws that affect Office detailed Wednesday is rated moderate and affects the Access Snapshot Viewer, a tool used to view Access databases without Access installed on a computer, Microsoft said in Security Bulletin MS03-038. (See: www.microsoft.com/technet/security/bulletin/MS03-038.asp)

Access Snapshot Viewer comes as part of all versions of Office, but is not installed by default. It is also offered online so users who do not have Access can still view Access databases, Microsoft said.

The flaw lies in an ActiveX control used by the viewer. To exploit the flaw, an attacker would have to lure a user to a Web page containing special code, Microsoft said.

Microsoft has a four-tiered system for rating security issues. Vulnerabilities that could be exploited to allow malicious Internet worms to spread without user action are rated critical. Issues that are rated important could still expose user data or threaten system resources. Vulnerabilities rated moderate are hard to exploit because of factors such as default configuration or auditing, or difficulty of exploitation, according to Microsoft.